Apparatuses having the above-mentioned features are shown in international patent application published with No. WO-A-9712724.
The embodiments described in such international application guarantee excellent metrological results and small forces of inertia and the standards of performance of the apparatuses with these characteristics, manufactured by the company applying for the present patent application, confirm the remarkable quality and the reliability of the applications.
In many numerical control grinding machines presently produced for working crankshafts, each piece to be worked is positioned on the worktable and rotated about its main rotation axis (i.e. the axis defined by the journal bearings), and during the rotation both journal bearings and crankpins are ground. As far as the crankpins are concerned, the proper working requires extremely accurate translation movements between the grinding-wheel slide and the worktable, synchronously with rotational movements of the shaft, under the control of the numerical control (NC) of the machine based on a proper working program that is the result of a numerical interpolation. Unavoidable imperfections in the dimensions or form deviation of the mechanical parts of the machine cause circularity or roundness deviations in the cylindrical surface of the ground workpiece. In order to correct such deviations (and considering that 2–3 μm is a typical value of tolerance for this kind of deviations, as required for crankshafts to be employed in cars), roundness of the worked crankpins must be checked, and the working program of the CN must be consequently corrected. Checking of the roundness of the crankpins is presently carried out by means of proper metrological apparatuses including a revolving table performing greatly accurate rotation movements, where the crankshaft is referred and fixed in such a way that the crankpin to be checked is substantially centred with respect to the rotation axis. A gauge having radial measuring axis detects the variations in correspondence of at least a transversal cross-section of the pin surface that is scanned in the course of a 360° rotation of the revolving table, with a proper sampling frequency. The detected variation values are processed to get the best-fit circumference, i.e. the circumference that best approximates the locus of the points corresponding to such values. Deviations of the detected values with respect to values of the best-fit circumference are calculated to define the roundness error of the checked surface, according to a well-known technique.
According to the presently used procedure, in order to check the roundness it is necessary to have a specific, costly and bulky apparatus, and to sequentially perform the following operations: remove the crankshaft to be checked from the grinding machine where it has been ground, position the crankshaft on the specific apparatus where careful operations are needed for a proper positioning and fixing on the revolving table, carry out the checking process, analyse the results, and manually correct the grinding program of the CN on the basis of such results. As a consequence, the involvement of properly instructed operators is needed to carry out the checking and the correction. Moreover, performing the above-mentioned operations negatively affects the working process, requiring not negligible interruptions, and appears in contrast with the even increasing requirements to continuously and timely check the production process.